1. Field of the Invention
The invention relates to a process for obtaining gaseous hydrocarbons, in particular gas mixtures similar to liquefied (petroleum) gas and natural gas. Here, as starting materials partially oxygen-containing hydrocarbons are converted over a catalyst into short-chain hydrocarbons. The process therefore provides a route for generating materials of value from secondary materials (which would have to be disposed of in an expensive manner).
2. Background Description
There have been many proposals in the past for converting fats or oils into fuels or other energy carriers into hydrocarbons. Thus, U.S. Pat. No. 1,960,951 describes a process for the catalytic conversion of vegetable oils in order to obtain liquid hydrocarbons, in addition to which volatile products are also formed. Activated carbon is used as catalyst here. The oil to be converted is pressurized by a compressed air compressor; it is subsequently passed in the liquid state through the heated activated carbon bed. This results in a kind of “cracking,” at least in the case of vegetable oils such as peanut oil.
DE 103 27 059 A 1 describes a process in which a fat- or oil-containing or fatty acid-containing starting material is brought into contact at a temperature of 150 to 850° C. in the absence of oxygen with an activated carbon fixed bed in a reactor. Here, the fats are dissociated and the fatty acids are decarboxylated and degraded to shorter-chain hydrocarbons.
DE 10 2005 023 601 A1 describes a variant of the process of DE 103 27 059 A1, in which the starting material is brought into contact with the activated carbon in the presence of water or a material which liberates water. Finally, DE 43 35 399 A1 describes a process for converting used oil or biooil into fuels similar to diesel oil, in which the vaporized starting material is brought into contact with catalysts containing perovskites at a temperature in the range from 350 to 500° C.
In all the above processes, essentially liquid hydrocarbons (having at least five carbon atoms) are produced. Shorter-chain hydrocarbons are only obtained as (undesirable) by-product which is at most used for heating the converted reactors.
Furthermore, it is known that biomass can be converted into syngas from which alkanes can be synthesized in the Fischer-Tropsch process. Hydrocarbons having various chain lengths are only subsequently built up again from the syngas (H2+CO). The Fischer-Tropsch process also has the disadvantage that it can be implemented economically feasibly only on a very large scale and is associated with a considerable energy consumption.
It is also known that ethanol can be produced from biogenic raw materials (cf., for example, S. Nordhoff, “Nachwachsende Rohstoffe in der chemischen Industrie—weg vom Öl?” Chemieingenieurtechnik 79 (5), 551 to 560). The pure hydrocarbon butane could be produced therefrom by hydrogenation. However, this makes little commercial sense.
Finally, it is also known that n-heptane can be converted by cracking over zeolite catalysts into hydrocarbon compounds having from 2 to 4 carbon atoms (N. Rane et al., “Cracking of n-heptane over Brönsted acid sites and Lewis acid Ga sites in ZSM-5 zeolites” Microporous and Mesoporous Materials 110, 2008, 279). However, this is a process in which only a single cracking step is required to convert the starting material into fragments having 3 or 4 carbon atoms.